It's the world's most exclusive, most powerful club and the price of admission is steep - US$12.7 billion ($15.2 billion) to be precise.

It has 85 members, who between them control assets totalling US$1.7 trillion - the same amount, according to a new Oxfam report, as that shared by the 3.5 billion people who make up the bottom half of the world's population.

The combined wealth of the 85 is believed to be greater than Australia's entire GDP, which was listed at US$1.56 trillion by the United Nations in 2012.

The singular wealth of American tech pioneer Bill Gates, who with US$77.1b is listed by Bloomberg as the world's richest man, exceeds the GDP of one of the US's nearest neighbours, Cuba.

The Microsoft co-founder is one of 34 US citizens among the world's top 85 billionaires, with his compatriots also taking seven spots in the top 10 on Bloomberg's rich list.

However, both Gates and fellow American rich-lister Warren Buffett (No 4 with US$58.9b) are also noted for their philanthropy, donating many millions each year to charitable causes.

She is ranked the world's eighth-richest woman, with a total of 10 females among the world's top 85 billionaires including Laurene Jobs, the widow of Apple co-founder Steve Jobs, at No 82 (US$13b).

The Oxfam report also warns the concentration of wealth is actually far greater than it can list, estimating that US$18.5 trillion - a figure greater than the GDP of the world's richest country, the US - sits unrecorded and hidden in offshore tax havens.